1. Field of the Invention
The present invention relates to an ink jet recording apparatus.
2. Description of the Related Art
An ink jet recording apparatus is an apparatus capable of recording a high-definition image on a recording medium by ejecting an ink droplet from an ejection orifice provided in a recording head. An ink jet recording apparatus used in a commercial printing field is required to conduct recording at a high speed. Thus, a full-line type recording head is introduced into the ink jet recording apparatus used for such a purpose. The full-line type recording head is provided with an ejection orifice array corresponding to the overall width of a recording medium by arranging plural recording element substrates in each of which a great number of ejection orifices are formed.
When high-speed recording is conducted, the number of ink droplets ejected from one ejection orifice per unit time increases. Thus, heat generated in the recording element substrate increases, and thermal unevenness between the respective recording element substrates is liable to occur. As a result, sticking of a coloring material due to evaporation of an ink occurs in the vicinity of an ejection orifice heated to a high temperature, and so there is a fear of ejection failure of an ink droplet or unevenness of the amount of the ink droplet. As a countermeasure for solving this problem, there is a method in which an ink is circulated up to the vicinity of an ejection orifice at all times to prevent the sticking of the ink. There is another method in which the temperature of an ink during circulation is kept constant to prevent the amount of an ink droplet from changing.
When the ink is circulated at a constant temperature, there is a possibility that water in the ink may be evaporated from each ejection orifice coming into contact with the air to increase the concentration of the ink during the circulation with time. There is thus a possibility that an image density may change. Therefore, it is necessary to periodically detect the ink concentration and correct the image density if the ink concentration has changed.
Patent Literature 1 (Japanese Patent Application Laid-Open No. 2010-014986) discloses a color image forming apparatus in which the density of an image for density detection, which has been recorded on a recording medium, is detected by a density sensor provided in a recording head to correct the density according to the result of the detection.
Patent Literature 2 (Japanese Patent Application Laid-Open No. 2005-186382) describes a method using, as a unit for detecting the concentration of an ink, a light detection unit which detects the intensity of transmitted light in a flow path through which the ink is circulated. Patent Literature 2 also describes a method of supplying a diluent or an ink of a predetermined concentration to an ink tank as a method for correcting the ink concentration.
According to the apparatus described in Patent Literature 1, it is necessary to periodically record the image for density detection on the recording medium. An image (printed article) that is an object of recording cannot be recorded during the recording of this image for density detection. Therefore, the correction of the image density impedes the efficiency of recording. In addition, when a pigment is used as a coloring material for an ink, the transmitted light detection unit described in Patent Literature 2 cannot be used. The reason for this is that light scattering caused by the pigment occurs to prevent the light from transmitting.